Does eddy subduction matter in the northeast Atlantic Ocean?

摘要

Mesoscale eddies are an important contributor to subduction in the Gulf Stream region and the Antarctic Circumpolar Current, but is eddy subduction also important in the relatively quiescent interior of the world's subtropical gyres? Observations from the Subduction Experiment of the northeast Atlantic do not have the spatial resolution necessary to calculate eddy subduction and answer this question. Regional numerical models can diagnose subduction, but their representativeness is unknown. Furthermore, water mass budgets in an open-ocean domain show that the simulated properties of subducted water directly depend upon uncertain open-boundary conditions and surface fluxes. To remedy these problems, a state estimate of the ocean circulation is formed by constraining an eddy-permitting general circulation model to observations by adjusting the model parameters within their uncertainty. The resulting estimate is self-consistent with the equations of motion and has the necessary resolution for diagnosing subduction. In the northeast Atlantic during 1991–1993, the time-variable circulation contributes less than 1 Sv of net subduction, while the total subduction is 4 Sv. Eddy volume fluxes of 40 m/yr in the North Equatorial Current and the Azores Current, however, are significant and rival the subduction by Ekman pumping locally. Furthermore, a state estimate at 1/6° resolution has 2–3 Sv more subduction in the density bands centered around σ = 24.0 kg/m3 and σ = 26.0 kg/m3 than a 2° state estimate. This result implies that the inability to accurately simulate mesoscale phenomena and surface fluxes in climate models would lead to an accumulation of errors in water mass properties over 10–20 years, even in the interior of the subtropical gyre.